In portable electric tools, for example, portable drills, screw drills and the like, it is desirable to reverse the direction of rotation of the motor to facilitate, for example, the loosening and removal of screws, or for rotating a thread-cutting tool out of a bore after cutting a thread therein. This is accomplished by reversing the polarity across the armature of the motor thereby changing the direction in which the current flows therethrough.
Trigger-switches for portable tools have been developed which perform this function. Such switches are constructed so that the reversal of polarity cannot occur until the switch has been completely opened. In one such trigger-switch arrangement which incorporates a reversing switch, the actuating lever of the reversing switch is disposed immediately above the trigger of the trigger-switch, and just below the motor compartment of the tool. The actuating lever of the reversing switch and the trigger are mechanically related so that the trigger remains inoperative, i.e., it cannot be moved, until the actuating lever is positioned to one side of the trigger to allow the motor to be energized to rotate in one direction, or until the actuating lever is positioned to the other side of the trigger to reverse the direction of rotation of the motor.
It is known that the mechanical neutral between the field coils of a motor does not necessarily coincide with the electrical neutral of the field generated by those coils when the armature is rotating. Such motors would provide optimum performance if the brushes engaging the armature's commutator are disposed on the electrical neutral, and it is for this reason that brushes in many motors are mounted in the motor housing so as to be offset from the mechanical neutral. When such a motor is operated in the opposite direction of rotation, the electrical neutral will shift to take on a position on the opposite side of the mechanical neutral. However, because the brushes have not been moved to the new electrical neutral, sparking and excessive brush and commutator wear will occur. In addition, the sparking action will cause electrical noise to be sent back into the power network supplying the electrical energy for the tool. Accordingly, to obtain optimum motor performance in the reverse direction of rotation, it is desirable to mechanically shift the brushes so that they can be located on the "reverse" electrical neutral on the opposite side of the mechanical neutral.
Arrangements such as that shown in U.S. Pat. No. 3,440,465 have been proposed for portable tools wherein the electrical switching operation to achieve rotation in the reverse direction is coordinated with the brush-shifting operation in such a manner that both operations occur in response to a single manual movement. However, these arrangements can be elaborate and require considerably electrical switching structure in the area of the commutator. In addition, a trigger-switch disposed at a location remote from the brush-shifting arrangement is still required to connect and disconnect electrical energy to the tool.
Another brush-shifting arrangement for a portable tool is disclosed in U.S. Pat. No. 2,764,705. This also incorporates an electrical switching mechanism to reverse the polarity of the motor. However, it also requires substantial structure in the area of the brushes and includes a mechanical interlock with the trigger-switch to ensure that the motor is not connected to the line voltage unless the brushes are in one of their two electrical neutral positions.
As indicated above, trigger-switches are known which incorporate a lever-actuated double-pole switch that can be wired to reverse the direction of current flow through the armature. Once the lever has been set, the operator can connect and disconnect electrical energy to the motor by operating the trigger-actuated switch by manually depressing the trigger.
The present invention is concerned with improving and further simplifying brush-shifting and reversing switch arrangements.
Accordingly, it is an object of the invention to provide a brush-shifting and trigger-switch arrangement for a portable electric tool wherein the brush-shifting mechanism is operatively coupled to the trigger-switch thereby making it unnecessary to provide an elaborate switching structure in the housing of the portable tool at the region of the brush-shifting mechanism.
It is a further object of the present invention to provide a brush-shifting and trigger-switch arrangement for a portable electric tool wherein the polarity of the lever-actuated reversing switch is determined by the angular position of the brushes with respect to the mechanical neutral plane of the motor, and wherein a manual shift of the brushes is prevented during the time that the trigger of the trigger-switch is depressed.
In the known combined trigger and reversing switches previously described, the lever of the lever actuated reversing switch is arranged outside the tool's housing. This brings with it at least two disadvantages to the operator. First, to accommodate the trigger-switch, openings are required to be formed in the housing of the portable tool wherein dirt and chippings can enter and cause the motor to malfunction; and, second, the lever requires considerable space which places the trigger of the trigger-switch a small, but some distance away from the main body of the tool housing, wherein the motor is mounted, causing the operator some discomfort when holding the tool and depressing the trigger with the index finger.
With regard to the second disadvantage mentioned above, a solution is suggested in U.S. Pat. No. 3,579,002 wherein the tool housing is provided with a small recess for accommodating the lever of the lever-actuated reversing switch.
The present invention is also concerned with improving combined trigger-switch and reversing switch arrangements.
Accordingly, it is, therefore, still a further object of the present invention to provide a trigger-switch arrangement for a portable tool which reduces or overcomes the two abovementioned disadvantages with known combinations of trigger and reversing switches.